This invention relates to apparatus for producing engraved printing forms, especially printing form cylinders.
The invention is an improvement on the apparatus disclosed in U.S. Pat. No. 3,816,698. That apparatus serves to produce screen-point printing forms, particularly printing form cylinders, by means of electron beam engraving. It provides an electron beam producing device which is located in a vacuum chamber that has a side which is open and faces the surface of the printing form and defines with this surface an air gap. The vacuum chamber and/or printing form can be moved relative to one another, while maintaining the gap. To permit engraving of the printing form up to the edge thereof without having to install the printing form in a vacuum chamber, the U.S. Patent in question provides an annular body which extends the edge of the printing form, for example, it has an outer circumferential surface that constitutes, when the ring is located adjacent an axial end of a printing form cylinder, an extension of the peripheral surface of the cylinder.
This concept of the aforementioned U.S. Patent is highly advantageous. However, its practical application is frequently hampered in that many of the printing form cylinders which are currently in use in the industry are of such a character that it is difficult to secure the annular body to an end face of the printing form cylinder. The peripheral surface of conventional printing form cylinders, and a portion of the end faces, is galvanically coated with a thin copper layer according to the Ballard method, for reasons which are well known to those conversant with this field. Since the edge of this copper layer is located on the axial end faces of the printing form cylinder, where the layer terminates, a precise mounting of the annular body on these axial end faces has in practice been found to be impossible. Moreover, even if such a mounting were not precluded by the aforementioned consideration, it would not be practical because the axial end faces of the printing form cylinder must be left free, since after the use of the cylinder it is necessary to peel off the copper skin, a process which starts at the axial end faces, in order to ready the cylinder for the application of a new copper layer. Special adaptations to overcome these problems are also not practical, since printers usually have a large number of printing form cylinders which represent a substantial monetary investment and must be re-useable time and time again, so that special-purpose adaptation is not practical.